Direct reduction processes for producing metallized iron from iron oxide pellets, lump ores, or similar materials which include iron oxide, are widely known and used in the steel industry. Direct reduction (DR) processes produce metals from their ores by removing associated oxygen from the ores at temperatures below the melting temperature of any of the materials involved in the process. The product obtained as a result of a direct reduction process is known as DRI or “Direct Reduced Iron”. The direct reduction process converts iron ore into a highly metallized iron product. The actual reduction of the iron ore in the direct reduction reactor is carried out in the presence of a reducing gas for example natural gas that comprises the reducing agents hydrogen (H2), and carbon monoxide (CO) which during contact reduces iron ore to metallic iron. Alternatively, reducing gas in the form of carbon monoxide can be produced by a gasifier by the combustion process that occurs when carbon reacts with oxygen which reaction also has the advantage of providing heat for the subsequent gasification reactions in the shaft furnace.
When the direct reduction reactions are carried out in furnaces below about 1000° C., the reducing agents usually are restricted to CO and H2. In the DRI process, the iron is reduced and the carbon dioxide, produced in the reduction reaction, is removed with the spent reducing gas commonly known as the reactor off-gas. The reactor off-gas includes unreacted hydrogen H2, unreacted carbon monoxide CO, carbon dioxide CO2 and water H2O in the form of steam. After cooling, the reactor off-gas is reprocessed to remove the carbon dioxide and enrich the hydrogen and carbon monoxide content before returning the enriched off-gas to the direct reduction reactor as an effective reducing gas. The refined product known as syngas (from synthesis gas) is a gas mixture containing varying amounts of carbon monoxide and hydrogen by gasification of the reactor off gas to a gaseous product with a heating value. Syngas is per definition a product containing primary of carbon monoxide, carbon dioxide and hydrogen. The reactor off gas contains significant levels of carbon dioxide and water which form during the reaction. Some schemes use a reforming step to provide more hydrogen and carbon monoxide and some schemes use a water gas shift step reaction to enhance the recycle gas to provide the refined high strength reducing gas known as syngas. All of the schemes must remove carbon dioxide from the reactor off gas to maintain the reduction process.
The direct reduction of iron ore, i.e. iron oxides mainly hematite Fe3O3, is accomplished by reduction of the iron ore by reaction with carbon monoxide, hydrogen and/or solid carbon through successive oxidation states to metallic iron. Typically, oxides of iron and carbonaceous material, e.g. coal, are charged into a furnace. Heat is supplied to the furnace by the combustion of fuel with air to generate, inter alia, carbon monoxide. As the solid material moves down the reactor it meets a stream of reducing gas flowing in the opposite direction and the material is reduced to metallic iron and recovered from the furnace. Exhaust and furnace gases are removed from the furnace through an exhaust gas line or duct. Direct reduction plants for producing direct reduced iron, known as DRI (sponge iron) or hot briquetted iron (pre-reduced materials useful as feedstock for iron and steel making), currently produce such products by contacting a reducing gas, composed principally of hydrogen and carbon monoxide, at effective reduction temperatures in the range from about 750° C. to about 1050° C., over a bed of particulate iron-containing material in the form of lumps or pellets. The gas streams used in direct reduction are often quantified on the basis of the ratio between the reducing agents and oxidizing agents defined as the mole ratio (H2+CO/H2O+CO2) of a gas mixture. This value is commonly referred to by the term “reducing ratio” or “R” value. The reducing ratio is a measure of the efficiency of a gas mixture when used as a reductant; the higher the reducing ratio the greater the efficiency.
During the years various processing schemes have been developed for generation of a high strength syngas from spent reducing gas or reactor off-gas, containing a high concentration of H2 and CO, for the direct reduction of iron (DRI) process and other processes. In the recent years, the necessity of rendering the steelmaking processes more efficient, more productive, and less wasteful, has become increasingly urgent, due to rising production costs (particularly energy costs) and also due to the increasing restrictions imposed upon steel plants for ecological reasons. The purpose of the present invention is therefore to achieve a process for production of direct reduced iron which in a simpler manner improves the reformation of the reactor off gas to produce syngas effluent high in hydrogen and carbon monoxide. Another purpose is to achieve a significant reduction of fossil fuels specific consumption, i.e. to lower the specific consumption of fossil primary energy and significantly reduce the specific CO2 emissions per ton produced DRI.
It is therefore an object of the invention to provide a process for production of direct produced iron by which the capacity of DRI production can be increased and which process at the same time enable to reduce the consumption of fossil fuels.
It is a further object of the present invention to achieve a recycling gas by cleaning and removal of carbon dioxide and steam from spent reducing gas in a process for the direct reduction wherein the recycled gas is essentially pure hydrogen in order to further improve reduction properties in the reactor.
It is a further object of the present invention to provide a process that reduces problems with metal dusting from the plant, particles and other undesirable components in the reactor off gas without increase use of water.
It is a further object of the present invention to provide a process that reduces use of water in particular as cooling medium during reformation of the reactor off gas to syngas. Cooling and reheating requires costly heat exchange equipment and associated large high temperature piping makes the process less attractive. It is also well known to use water in scrubbers for dust removal.
Other objects of the invention will be pointed out in this specification or will be evident to those skilled in the art.